Method and apparatus for controlling a torque converter clutch

ABSTRACT

A method and apparatus are provided for controlling actuation of a clutch device for a torque converter operative to transmit torque between an engine and a transmission. The method comprises controlling actuation of the clutch device effective to maintain an engine speed within a predetermined speed range when a transmission input speed is less than a threshold; and, controlling actuation of the clutch device effective to maintain slippage across the torque converter substantially at a predetermined slippage level when the transmission input speed is greater than the threshold.

TECHNICAL FIELD

This invention pertains generally to torque transmission devices, andmore particularly to controlling a clutch for a torque converter device.

BACKGROUND OF THE INVENTION

A torque converter is typically placed between an internal combustionengine and an automatic transmission device and operative to transmittorque therebetween, using an impeller and a turbine device in a fluidicmedium. A torque converter clutch typically comprises a pressurizedfluid-actuated friction device engageable to mechanically couple theimpeller, receiving input from the engine, and the turbine, having anoutput to the transmission. In a typical application, the clutch can befully released, actuated in a slip mode, and fully engaged, i.e. locked.When the clutch is fully released, there is unrestrained slippagebetween the impeller and the turbine, and torque is transmittedtherebetween based upon the flow of hydraulic fluid between the impellerand the turbine. When the clutch is actuated in the slip mode, torque istransmitted between the impeller and the turbine through the flow ofhydraulic fluid therebetween and controlling pressure of hydraulic fluidto the actuated clutch, and typically there is a difference inrotational speeds between the impeller and the turbine, i.e., a relativespeed. When the clutch is fully released, or actuated in the slip mode,torque perturbations between the engine and the transmission resultingfrom either engine operation or driveline dynamics are absorbed in thefluid of the torque converter.

When the clutch is fully engaged, the rotational speeds of the impellerand the turbine are the same, and torque is transmitted between theimpeller and the turbine through the actuated torque converter clutch.When the torque converter clutch is fully engaged, a range of enginetorque perturbations or torsionals, typically in the range of 2 to 6 Hz,are passed directly through the clutch to the vehicle drivetrain,producing pulsations therein when not properly damped. Other torsionals,typically those above about 20 Hz, are absorbed in a torsional damperdevice, which is an element of the torque converter. Thus, the action ofcompletely locking the torque converter clutch is often restricted tospecified vehicle operating conditions to minimize the effects on noise,vibration and harshness (NVH). As a result, potential efficiency gainsafforded by fully engaging the torque converter clutch are only realizedover a portion of the range of vehicle operation.

To overcome the disadvantages of torque converter clutch engagement, ithas been proposed to operate the clutch in a slipping mode wherein apredetermined amount of slippage between the torque converter impellerand turbine is permitted for regulating the torque capacity of theclutch. In any such system, the objective is to isolate engine torqueperturbations in the torque converter while passing steady state enginetorque at a slip rate that provides improved torque converterefficiency, leading to improved fuel economy. Previous control systemsproposed to manage clutch slippage have been disclosed, for example, inU.S. Pat. No. 4,582,185 to Grimes et al., issued Apr. 15, 1986, and U.S.Pat. No. 5,484,354, to Vukovich, et al., issued Jan. 16, 1996, eachwhich is assigned to the assignee of the present invention. The adventof cylinder deactivation systems has further emphasized a need toeffectively manage operation and control of a torque converter clutch ina modern powertrain system.

There is a need to expand range of usage of the torque converter clutchin order to gain efficiency benefits therefrom without adverselyaffecting driveability.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention, there is provided amethod, preferably executed as a computer program in a control modulefor a powertrain system for controlling actuation of a clutch device fora torque converter operative to transmit torque between an engine and atransmission. The method comprises controlling actuation of the clutchdevice effective to maintain an engine speed within a predeterminedspeed range when a transmission input speed is less than a threshold;and, controlling actuation of the clutch device effective to maintainslippage across the torque converter substantially at a predeterminedslippage level when the transmission input speed is greater than thethreshold.

Benefits of operating an embodiment of the invention described hereincomprise achieving a wider operating range for the torque converterclutch without sacrificing driveability and pleasability of vehicleoperation. Furthermore, operating the system at a fixed slippageprovides driveline damping and decouples engine oscillations from thedriveline.

The invention will become apparent to those skilled in the art uponreading and understanding the following detailed description of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement ofparts, an embodiment of which is described in detail and illustrated inthe accompanying drawings which form a part hereof, and wherein:

FIG. 1 is a schematic diagram of an exemplary system, in accordance withthe present invention;

FIG. 2 is an algorithmic flowchart, in accordance with the presentinvention;

FIG. 3 is a graphical depiction of results from a prior art system;

FIG. 4 is a graphical depiction of results, in accordance with thepresent invention;

FIG. 5 is a graphical depiction of results from a prior art system; and,

FIG. 6 is a graphical depiction of results, in accordance with thepresent invention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

Referring now to the drawings, wherein the showings are for the purposeof illustrating the invention only and not for the purpose of limitingthe same, FIG. 1 depicts an engine 10, transmission 30, driveline 40,and control system which have been constructed in accordance with anembodiment of the present invention. The exemplary engine 10 preferablycomprises any one of various multi-cylinder internal combustion engineconfigurations controlled by an engine control module (ECM) 5 based upona torque request T_(O) _(—) _(REQ) input from a user interface 13 whichprovides one or more operator inputs, e.g. from a throttle pedal deviceand a brake pedal device. A rotatable output shaft of the engine isconnected to an input shaft of a fluidic torque converter 20 which ispreferably disposed within a housing of transmission 30.

The torque converter 20 includes a torque converter clutch (TCC) 25,which is operative to transmit torque input from the input shaft fromthe engine, when engaged. As depicted in FIG. 1, engine power output,depicted as engine rotational speed, N_(E), measured in revolutions perminute (rpm) and engine torque, T_(E), measured in Newton-meters (N-m)can be transmitted across either or both the torque converter 20 and theTCC 25 to the input shaft of the transmission 30. The input shaft totorque converter 20 is connected to an impeller or input member (notshown) of the torque converter 20. A turbine or output member (notshown) of the torque converter 20 is rotatably driven by the impeller bymeans of fluid transfer therebetween, and connects to and rotatablydrives a shaft input to the transmission 30, which, as depicted in FIG.1, has inputs of transmission input speed, N_(I) and torque, T_(I). Thetorque converter clutch (TCC) assembly 25 preferably comprises ahydraulically-actuated clutch device that is selectively controlled toengage the impeller and the turbine. An exemplary TCC assembly 25 isdescribed in detail in U.S. Pat. No. 5,484,354, to Vukovich, et al.,issued Jan. 16, 1996, which is assigned to the assignee of the presentinvention. The TCC is preferably controlled by a transmission controlmodule (TCM) 15 operative to generate a pulse-width-modulated (PWM)signal having a variable duty cycle effective to control hydraulicpressure thereat.

In operation, the torque converter 20 and TCC 25 typically operate asfollows. The TCC can be fully released, actuated in a slip mode, andfully engaged, i.e. locked. When the clutch is fully released, there isunrestrained slippage between the impeller and the turbine, and torqueis transmitted therebetween based upon the flow of hydraulic fluidbetween the impeller and the turbine. When the TCC is actuated in a slipmode, the TCC 25 is actuated but there is slippage between the impellerand the turbine, with a resulting difference in rotational speedsbetween the impeller and the turbine. Torque is transmitted between theimpeller and the turbine through the flow of hydraulic fluid and throughthe actuated TCC 25, with slippage in the system which absorbs engineand driveline perturbations. When the TCC 25 is locked, the rotationalspeeds of the impeller and the turbine are the same, and torque istransmitted between the impeller and the turbine through the actuatedclutch. The TCC 25 is controlled by a PWM signal from the TCM 15, suchthat when the PWM duty cycle is relatively low, the clutch pressure islow, and the torque converter functions as a normally fluidic pumpdevice, as previously described. When the PWM duty cycle is increased,hydraulic pressure increases, increasing clutch pressure andmechanically engaging the impeller and the turbine devices, with a levelof slippage therebetween (N_(E)-N_(I)) based upon the clutch pressure,engine torque and speed, and other operating conditions. Overallactuation of the TCC 25 is generally known and not discussed in detailherein.

The output from the torque converter 20, comprising torque T_(I) androtational speed N_(I), is transmitted through the turbine/impeller andthe TCC 25, is input through a shaft to the transmission 30. Thetransmission 30 comprises a gear set suitable for providing a pluralityof fixed gear ratios between the torque converter output shaft and anoutput shaft of the transmission. The transmission 30 preferablyincludes a hydraulic pump and circuit operative to supply pressurizedhydraulic fluid to various devices in the transmission to effectoperation of the transmission and the torque converter. The output shaftof the transmission is characterized by an output speed, N_(O), andoutput torque T_(O), and is operatively connected to driveline 40 fordelivering tractive torque to one or more vehicle wheels, characterizedby a vehicle speed parameter, V_(SS).

The system includes sensing devices operative to sense operator demands,and operating conditions of the engine and transmission devices.Operator demands, depicted as the torque request T_(O) _(—) _(REQ) inputfrom the user interface 13 in FIG. 1, typically comprise demands fortorque in the form of acceleration and braking using inputs from anaccelerator pedal and a brake pedal. Engine operating conditions aredetermined using sensing devices are installed on the engine to monitorphysical characteristics and generate signals which are correlatable toengine and ambient parameters, specifically an engine operating point.The engine operating point comprises a measure of engine crankshaftspeed output to the transmission (N_(E)) and load (MAP), measurableusing, e.g., an intake manifold pressure sensor or a mass air flowsensor. Each of the sensing devices is signally connected to the ECM 5to provide signal information which is transformed by the ECM toinformation representative of the respective monitored parameter. It isunderstood that this configuration is illustrative, not restrictive,including the various sensing devices being replaceable withinfunctional equivalent devices and algorithms and still fall within thescope of the invention. The transmission 30 includes an output speedsensor, typically a variable reluctance transducer, operative to monitorrotational speed, N_(O), of the output shaft, from which input speed,N_(I), of the transmission is determined based upon the specific gearratio (GR) at which the transmission 30 is operating. Alternatively, asensor can be mechanized in a system to directly monitor transmissioninput speed, N_(I).

The control system for operation of the invention described hereincomprises elements of an overall vehicle control system, preferablyexecuted as a distributed control module architecture to providecoordinated system control. The ECM 5, TCM 15 and the user interface 13are each signally connected via a local area network (LAN) 6, which isoperative to provide structured signal communication between the variouscontrol modules. The ECM and the TCM are each operable to synthesizepertinent information and inputs from the aforementioned sensingdevices, and execute algorithms to control various actuators to achievecontrol targets, including such parameters as fuel economy, emissions,performance, driveability, and protection of hardware. The ECM and TCMare preferably general-purpose digital computers each generallycomprising a microprocessor or central processing unit, storage mediumscomprising read only memory (ROM), random access memory (RAM),electrically programmable read only memory (EPROM), high speed clock,analog to digital (A/D) and digital to analog (D/A) circuitry, andinput/output circuitry and devices (I/O) and appropriate signalconditioning and buffer circuitry. A set of control algorithms,comprising resident program instructions and calibrations, is stored inROM and executed to provide the respective functions of each computer.Algorithms are typically executed during preset loop cycles such thateach algorithm is executed at least once each loop cycle. Algorithmsstored in the non-volatile memory devices are executed by one of thecentral processing units and are operable to monitor inputs from thesensing devices and execute control and diagnostic routines to controloperation of the respective device, using preset calibrations. Loopcycles are typically executed at regular intervals, for example each3.125, 6.25, 12.5, 25 and 100 milliseconds during ongoing engine andvehicle operation. Alternatively, algorithms may be executed in responseto occurrence of an event.

Referring now to FIG. 2, the invention comprises a method, preferably asone or more algorithms which are ongoingly executed primarily in the TCM15, effective to control actuation and operation of the TCC 25 duringongoing vehicle operation. The method includes determining parametricvalues for the engine operating point, engine speed, i.e., transmissioninput speed, and, an operator demand for torque. The TCC is engaged andactuated to a control level, based upon the operator demand for torqueand the engine operating point, preferably using a feed-forward controlalgorithm. When the transmission input speed is less than a threshold,actuation of the clutch device is controlled in a manner effective tomaintain the engine speed greater than a minimum speed level. When thetransmission input speed is greater than the aforementioned threshold,actuation of the clutch device is controlled effective to maintainslippage across the torque converter substantially at a preset slippagelevel. This is described in detail hereinafter.

Referring again to FIG. 2, the TCM commands operation in TCC slip mode(Block 110) to engage the TCC 25, typically based upon operatingconditions in the engine, transmission and driveline. Operatingconditions are monitored (Block 112), typically including engine speedN_(E), and torque T_(E), transmission input speed, N_(I), output speedN_(O), gear ratio GR, and hydraulic pressure PR_HYD, and operator inputsincluding operator torque request T_(O) _(—) _(REQ). Slip across the TCCis determined, based upon a difference between the engine speed N_(E),and the transmission input speed N_(I) (Block 114). The TCC is actuatedby controlling flow of pressurized hydraulic fluid to engage the TCC tothe torque converter using a feed-forward control scheme. Thefeed-forward control scheme engages the TCC by determining a TCCcommand. The TCC command is in the form of a PWM control signal,TCC_PWM, based upon the requested torque T_(O) _(—) _(REQ) and enginetorque T_(E), which is determined from the engine operating pointdescribed above (Block 116). The feed-forward control scheme preferablycomprises a known control scheme which uses proportional and integralelements in an iterative operation to achieve the determined PWM controlsignal, TCC_PWM and engage the TCC.

When the TCC is engaged by actuation thereof, the control mode switchesto an APPLY mode, wherein the TCC command TCC_PWM is determined basedupon the feed-forward control, adjusted using a feedback control schemeas now described. In the APPLY mode, the type of feedback control isdetermined based upon transmission input speed N_(I) (Block 118). Whenthe transmission input speed is above a threshold (typically a range of900 to 1100 rpm), the PWM control signal, TCC_PWM is adjusted based uponmagnitude of slippage across the torque converter, determined as shownin block 114. When the transmission input speed is below the threshold,the PWM control signal TCC_PWM is controlled based upon magnitude ofengine speed N_(E). Each feedback control scheme is now described.

When the transmission input speed is below the threshold, actuation ofthe torque converter clutch is controlled effective to maintain theengine speed greater than a minimum speed level, within an allowablerange or deadband (DB). The engine speed N_(E) is compared to a minimumengine speed, N_(E) _(—) _(MIN) (Block 130). When the engine speed N_(E)is greater than the minimum engine speed, N_(E) _(—) _(MIN), by anamount greater than the deadband, the duty cycle of the PWM controlsignal TCC_PWM is increased (Block 132) by a fixed amount, e.g. 1%, orby an amount that is determined based upon the difference between theengine speed N_(E) and the minimum engine speed, N_(E) _(—) _(MIN). Whenthe engine speed N_(E) is less than the minimum engine speed, N_(E) _(—)_(MIN), by an amount greater than the deadband, the duty cycle of thePWM control signal TCC_PWM is decreased (Block 134) by a fixed amount,e.g. 1%, or by an amount that is determined based upon the differencebetween the engine speed N_(E) and the minimum engine speed, N_(E) _(—)_(MIN). The intent of this engine speed control scheme is to controlapplication of the torque converter clutch to maintain engine speedwithin an allowable range of the minimum speed. The minimum enginespeed, N_(E) _(—) _(MIN) is determined based upon operatingcharacteristics of the engine and transmission including the hydraulicpump. In the application described herein, a parametric value for theminimum engine speed, N_(E) _(—) _(MIN) is set in the range of 900 to1100 rpm when operating in the slip mode, in order to provide asufficient amount of power to the hydraulic pump to have sufficienthydraulic pressure in the transmission for effective operation thereof.Thus, for purposes of illustration, a possible calibration can comprisea parametric value for N_(E) _(—) _(MIN) of 1000 rpm, with a deadbandvalue of 100 rpm.

When the transmission input speed is above the threshold (Block 118),actuation of the TCC is controlled effective to maintain slippage acrossthe torque converter substantially at a desired slippage level,typically measured as in a difference in rpm (ΔRPM). This includescomparing the determined slip across the TCC to a desired slip,SLIP_DES. (Block 120). A deadband slip, DB, comprising a predeterminedhysteresis value of slip (measured in ΔRPM), is included to allow forerrors and delays related to mechanical, hydraulic, electrical, andmeasurement systems operations. When a difference between the determinedslip and the desired slip is greater than the deadband slip, the systemdetermines that the slip is greater than desired, and the duty cycle ofthe PWM control signal TCC_PWM is increased (Block 122) by a fixedamount, e.g. 1%, or by another suitable amount. When the determined slipis less than the desired slip by an amount greater than the deadbandslip, the system determines that the slip is less than desired, and theduty cycle of the PWM control signal TCC_PWM is decreased (Block 124) bya fixed amount, e.g. 1%, or by another suitable amount. In thisapplication, a preferred level of slippage is in the range of about 40to 50 RPM, with a deadband of about 5 RPM. The level of slippagecomprises an optimal value, wherein too much slippage can result in lossof engine torque through the torque converter, and too little slippagecan result in transmission of unwanted engine or driveline perturbationsand resonances to the vehicle chassis and the operator.

Referring now to FIGS. 3-6, graphical depictions of data are presentedwhich demonstrate effectiveness of operating the invention when it isexecuted as an algorithm in a vehicle system equipped with a four-speedautomatic transmission. Each depiction comprises operation during launchof a vehicle, and successive progression through gears 1 through 4, withtime-based data comprising engine speed N_(E) (in RPM), transmissioninput speed or turbine speed N_(I) (in RPM), slip (in RPM), PWM controlsignal TCC_PWM (in percent), transmission output torque T_(O) (in N-m)and vehicle speed Vss, in kilometers per hour (KpH). FIG. 3 depictsresults from a conventional prior art system operated with input ofthrottle position of 5%, i.e., a low operator demand for acceleration.FIG. 4 depicts a system equipped with an embodiment of the invention,operated similarly to that described with reference to FIG. 3, withoperator input of throttle position of 5%. The results demonstrate thatthe magnitude of slip across the torque converter is substantially lesswith the embodiment of the invention under these conditions, whereinslip is controlled to a desired slip level of 40 RPM during operation ineach of second and third gears. Furthermore, as identified by the areamarked as ‘A’ in FIG. 4, engine operation is maintained at about 900 rpmin fourth gear, and the magnitude of slippage is steadily decreased,thus demonstrating operation of the slip control portion of thealgorithm described above.

FIG. 5 depicts results from a conventional prior art system operatedwith input of throttle position of 20%, i.e., a moderate operator demandfor acceleration. FIG. 6 depicts a system equipped with an embodiment ofthe invention, with the operator input of throttle position of 20%. Theresults demonstrate that the magnitude of slip across the torqueconverter is substantially less with the embodiment of the inventionunder these conditions, with slip substantially close to the desiredslip during operation in each of the second, third and fourth gears.Furthermore, as identified by the area marked as ‘B’ in FIG. 6, engineoperation is maintained at about 1100 RPM in fourth gear, and themagnitude of slippage is steadily decreased to the desired slip, atwhich time the engine speed begins to increase, again demonstratingoperation of the slip control portion of the algorithm described above.

It is understood that modifications in the hardware are allowable withinthe scope of the invention. The invention has been described withspecific reference to the embodiments and modifications thereto. Furthermodifications and alterations may occur to others upon reading andunderstanding the specification. It is intended to include all suchmodifications and alterations insofar as they come within the scope ofthe invention.

1. Method for controlling actuation of a clutch device for a torqueconverter operative to transmit torque between an engine and atransmission, comprising: monitoring a transmission input speed;comparing the monitored transmission input speed to a threshold inputspeed; when the transmission input speed is less than the thresholdinput speed, controlling an engine speed based upon a minimum enginespeed comprising: comparing the engine speed to the minimum enginespeed; and controlling actuation of the clutch device based upon aresult of the comparing the engine speed to the minimum engine speed;and when the transmission input speed is greater than the thresholdinput speed, controlling a torque converter slip based upon a desiredslip comprising: comparing the torque converter slip to the desiredslip; and controlling actuation of the clutch device based upon a resultof the comparing the torque converter slip to the desired slip.
 2. Themethod of claim 1, wherein controlling actuation of the clutch devicebased upon the result of the comparing the engine speed to the minimumengine speed comprises decreasing actuation pressure of the clutchdevice when the engine speed is substantially less than the minimumengine speed.
 3. The method of claim 2, wherein controlling actuation ofthe clutch device based upon the result of the comparing the enginespeed to the minimum engine speed further comprises increasing actuationpressure of the clutch device when the engine speed is substantiallygreater than the minimum engine speed.
 4. The method of claim 3, whereinthe minimum engine speed comprises an engine speed sufficient to providepower to a hydraulic pump for effective hydraulic pressure in thetransmission.
 5. The method of claim 2, wherein the minimum engine speedcomprises a value within a range from 900 revolutions per minute to 1100revolutions per minute.
 6. The method of claim 1, wherein controllingactuation of the clutch device based upon the result of the comparingthe torque converter slip to the desired slip comprises increasingactuation pressure of the clutch device when the torque converter slipis substantially greater than the desired slip.
 7. The method of claim6, wherein controlling actuation of the clutch device based upon theresult of the comparing the torque converter slip to the desired slipfurther comprises decreasing actuation pressure of the clutch devicewhen the torque converter slip is substantially less than the desiredslip.
 8. The method of claim 7, wherein the desired slip comprises afixed parametric difference between the engine speed and thetransmission input speed.
 9. The method of claim 7, wherein the desiredslip comprises a slippage level incremented by a deadband slippagelevel.
 10. The method of claim 1, wherein controlling actuation of theclutch device based upon the result of the comparing the torqueconverter slip to the desired slip comprises maintaining actuationpressure of the clutch device when the torque converter slip is withinan allowable range of the desired slip.
 11. The method of claim 1,further comprising engaging and actuating the clutch device based uponan operator demand for torque and an engine operating point.
 12. Themethod of claim 11, wherein engaging and actuating the clutch devicebased upon the operator demand for torque and the engine operating pointcomprises executing a feed-forward control scheme to control actuationof the clutch device.
 13. The method of claim 1, wherein the thresholdinput speed is selected based upon a predetermined speed range.
 14. Themethod of claim 1, wherein the threshold input speed comprises a valuewithin a range from 900 revolutions per minute to 1100 revolutions perminute.